1. Field of the Invention
The present invention relates to an image processing apparatus and a recording/reproducing apparatus and, more particularly, a recording/reproducing apparatus for recording information on a recording medium and reproducing the recorded information as image information using a signal obtained by a physical interaction between the recording medium and a probe electrode upon two-dimensional scanning of the probe electrode relative to the recording medium, and an image processing apparatus suitable for this recording/reproducing apparatus.
2. Related Background Art
In recent years, the recording capacity of data required for a recording/reproducing apparatus tends to increase. To meet this increase, the size of the unit of recording is reduced, and the density of the unit of recording must be increased. For example, in a digital audio disk using optical recording, the size of the unit of recording is reduced to about 1 .mu.m.sup.2.
On the other hand, a scanning tunnel microscope (STM) capable of allowing direct observation of the electron structure on or near the surface of a material has been developed and has the following advantages, so that a variety of applications are expected.
(1) A real spatial image can be measured at a high resolution regardless of the monocrystalline and amorphous materials.
(2) A low-power observation can be performed without damaging a recording medium by a current.
(3) The microscope can be operated in an ultra high vacuum, air, and a solution.
(4) The microscope can be used for a variety of materials.
The scanning tunnel microscope utilizes the flow of a tunnel current (i.e., a signal obtained by a physical interaction between a probe electrode (metal probe) and a conductive material) between the probe electrode and the conductive material while a voltage is being applied between the probe electrode and the conductive material. The tunnel current is very sensitive to a change in distance between the probe electrode and the conductive material. When the probe electrode is two-dimensionally scanned relative to the surface of the conductive material so as to keep the average distance between the probe electrode and the conductive material or the tunnel current constant, thereby obtaining surface information of the real space of the conductive material. At this time, the resolution in the longitudinal direction of the conductive material is 10 nm or less.
When the principle of this scanning tunnel microscope is applied, a material (a thin film layer of .pi. electron organic compound or chalcogen compound) having a memory effect for voltage and current switching characteristics is used as a recording medium to allow recording/reproduction of information in recording units of 0.01 .mu.m.sup.2 or less. In addition, a technique for changing the surface shape/state of a recording medium using an electromagnetic wave such as an electron beam or light is used to allow information recording/reproduction at a recording density almost equal to that of the state-of-the-art optical recording although the unit of recording increases due to the limitations of the degree of focusing of an electron beam or the like.
In a conventional recording/reproducing apparatus to which the principle of the scanning tunnel microscope is applied, when high-density recording/reproduction is to be performed on the entire surface (recording surface) of a recording medium having a predetermined area, the S/N ratio of the reproduced signal decreases and the error rate increases due to the three-dimensional pattern formed on the recording surface. The three-dimensional pattern avoids an increase in recording capacity.
To reduce this physical three-dimensional pattern on the surface of the recording medium, a method of growing a monocrystalline Au thin film on the cleavage surface of mica or the like to obtain a smooth surface can be used. Even if a recording medium obtained by this method is used, however, the shape of a recording bit is changed by a step or ridge on the atomic order through a three-dimensional pattern having a size almost equal to that of a recording bit on the order of 10 nm or less in diameter. As a result, the S/N ratio of the reproduced signal decreases, and the error ratio increases.
Filtering in a frequency range belonging to a linear filter may be used as a method of reducing noise of the reproduced signal. However, this method is effective only if the frequency bands of a signal and noise in the reproduction mode are known and can be linearly separated. Although the frequency bands for individual shapes are known, the positions of recording bits are unknown. In addition, the step or ridge has a portion (i.e., an edge) having a large differential coefficient on the spatial coordinate system. For this reason, these frequency components extend through the entire frequency range. At the same time, the compact recording bit frequency components spatially extend to the entire range. Therefore, these frequency components cannot be separated into bands on the frequency space. In addition, the position of the step or ridge cannot be specified, and it is therefore difficult to perform extraction using template matching of a power spectrum.
On the other hand, a technique belonging to the field of nonlinear filters is available as a conventional image processing technique having spatial correlation as its most important factor. According to this technique, an extraction image obtained by extracting the step or ridge from an image is prepared, and the extraction image is subtracted from the original image to remove the step or ridge. Since the technique for extracting only the step or ridge from the image, however, has a repetitive and serial processing form and strongly depends on the number of pixels, this technique is very disadvantageous in a recording/reproducing apparatus whose read rate is most important.